The present invention relates to a thermally responsive switch wherein a movable contact is engaged with and disengaged from a fixed contact by deforming a bimetallic or trimetallic thermally responsive element to thereby open and close an electric circuit, and more particularly to such a thermally responsive switch suitable for protection of electric motors of enclosed compressors used in refrigerators, room air conditioners and the like.
The thermally responsive switches of the above-described type are disclosed as "MINIATURE ELECTRICAL SWITCH" in Japanese Published Patent Application No. 45-40818 or as "MOTOR PROTECTOR AND METHOD OF MAKING THE SAME" in Japanese Published Patent Application No. 49-24744. However, these switches each have a problem of calibration of the switch to set an operative temperature of a thermally responsive element.
Generally, in the thermally responsive switches, a shallow dish shaped portion is formed in the central portion of the bimetallic thermally responsive element by way of drawing so that the same reverses its curvature by snap action in response to heat. In the solid state that the thermally responsive element undergoes no restrictive force at the free end, the thermally responsive element reverses its curvature by snap action at a predetermined temperature, for example, 140.degree. C. and thereafter, recovers to the former state at a predetermined temperature, for example, 80.degree. C. While, in the switch mechanism wherein the thermally responsive element is secured at one end to a stationary member and carries a movable contact at the other or free end and wherein the movable contact is engaged with a fixed contact secured to a stationary member, with a predetermined contact pressure at the normal temperature, the thermally responsive element is more liable to reverse its curvature as the contact pressure between movable and fixed contacts is increased. For example, where the thermally responsive element reverses its curvature by snap action at 140.degree. C. in the above-mentioned solid state, the thermally responsive element reverses its curvature at 130.degree. C. to thereby disengage the movable contact from the fixed contact when the contact pressure is increased, thereby obtaining an ideal thermally responsive switch in which the on-off operation thereof is instantaneously performed without any creeping. In the calibration of the thermally responsive switch, when the operative temperature of the thermally responsive element in the solid state or in the state that the movable contact undergoes no force imposed by the fixed contact to move it to the OFF position is in the range between 135.degree. C. and 150.degree. C. in accordance with a number of samples, the operative temperature at which the movable contact is disengaged from the fixed contact by snap action is adjusted to the range of 130 5.degree. C. .+-.5.degree. C., for example. This work is referred to as an operative temperature calibration step. More specifically, in the operative temperature calibration step, either the member on which the fixed contact is secured or the member on which the fixed end of the thermally responsive element is secured is bent so that the contact pressure is impressed. Since the operative temperatures of the solid thermally responsive elements differ from one another, an amount of bending deformation differ from one thermally responsive element to another, which causes an amount of the so-called springback to differ from one thermally responsive switch to another. Consequently, the calibrated operative temperatures of the thermally responsive elements are rendered inaccurate.